Battery Backup Runtime Calculator

Work out how long a 12, 24 or 48 V battery bank will run a load, and how much capacity you need to hit a target runtime. Includes discharge depth, inverter losses and a C-rate check.

Runtime is not printed on the battery. It falls out of the energy stored, how much of it you are willing to use, the inverter losses and the load. Fill in the bank and the load, and the page returns hours, usable energy and the capacity you need for a target runtime.

The battery bank

Use the nominal voltage of the bank (12, 24 or 48 V). Capacity is the Ah printed on one battery; parallel strings add capacity, series strings add voltage.

Bank voltage Capacity per battery (Ah) Parallel strings Usable depth of discharge Inverter efficiency Load (W) Target runtime (hours)

Backup runtime –

Usable energy –

Bank capacity –

Discharge current –

C-rate –

Capacity for target –

How this was worked out

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Runtime for a 12 V 100 Ah bank at 50% depth

Load	Runtime	Current
50 W	10 h 48 min	4.6 A
100 W	5 h 24 min	9.3 A
200 W	2 h 42 min	18.5 A
300 W	1 h 48 min	27.8 A
500 W	1 h 05 min	46.3 A
1000 W	0 h 32 min	92.6 A

Four rules before you buy

Lead-acid batteries should not go below 50% depth of discharge; the last 20% costs a lot of cycle life. Lithium can be taken to 80–100%.
The inverter eats 5–15%. Use the efficiency on its datasheet, not the marketing number.
A 100 Ah battery is rated at the 20-hour rate. At high current the delivered capacity drops, so treat the result as an upper bound.
Keep the discharge current under about 0.2C for lead-acid and 0.5C for lithium; above that, voltage sag and heat shorten life.

Practical notes

Count only what you actually need to run: fridge, lights, router, pump. A 3 kW kettle will flatten a 100 Ah bank in minutes.
Add a 20% margin on the runtime you need, because cold weather and ageing cut capacity.
Series strings add voltage but keep the Ah; parallel strings add Ah but keep the voltage. Do not mix old and new batteries in one string.

Three common mistakes

Using the battery Ah × voltage as usable energy. A 12 V 100 Ah battery holds 1,200 Wh, but only about 540 Wh is usable at 50% depth and 90% efficiency.
Forgetting the inverter. A 300 W AC load draws more than 300 W from the battery.
Sizing to the peak surge. Motors draw 2–3 times their running watts for a second; the bank must survive the surge, the runtime is set by the running load.